The present invention relates to a fuel assembly and an upper tie plate thereof, and specially to the fuel assembly and the upper tie plate thereof which are preferable for effective utilization of fissile material and achieving high burnup in boiling water reactors.
Improvement of fuel economy is able to achieved by increasing the degree of burnup of the fuel. For increasing the degree of burnup of the fuel, enrichment of uranium 235 in the fuel pellet may be increased. But, increasing of the enrichment without increasing of the moderator to fuel atom number density ratio (H/U ratio) causes hardening of a neutron spectrum. Therefore, an finite multiplication factor of the fuel assembly does not become the maximum value at the enrichment.
FIG. 1 illustrates change of the relation between the H/U ratio and the infinite multiplication factor depending on increasing of the enrichment. For obtaining large infinite multiplication factor with a constant enrichment, it is necessary to achieve the most proper H/U ratio corresponding to the enrichment. That is, when the enrichment is increased in order to improve the fuel economy, the most proper H/U ratio is increased, and accordingly it becomes necessary to increase the number of water rods or to increase a horizontal cross sectional area of the water rods.
And, when the enrichment is increased, power peaking in radial direction of the fuel assembly is increased and linear power density of fuel rod becomes large, and consequently the fuel rod is exposed to a more severe condition.
Further, distribution of voids in axial direction of the reactor core is small at the lower end portion of the reactor core, and is large from the middle to the upper end portion of the reactor core. Therefore, as burning of the fissile material at the upper region of the fuel assembly is retarded, the concentration of uranium 235 becomes higher relatively than that in the other portion. And by effect of the void, fissile plutonium is produced and built up at the upper region of the fuel assembly. According to the reason mentioned above, power peaking becomes high at the upper portion in axial direction of the fuel assembly. As the increasing of the enrichment relates also to the increasing of power peaking in the axial direction, linear power density of the fuel rod becomes large as well, and the fuel rod is exposed to a more severe condition.
On the other hand, a flow rate spectral shift operation of a nuclear reactor is currently considered, in which the void fraction is changed greatly by operating the nuclear reactor with smaller flow rate (the flow rate of the coolant which flows through the reactor core) in the reactor core than the designed flow rate value at the beginning of operation cycle and with larger flow rate in the reactor core than the designed flow rate value at the end of the operation cycle, and fissile plutonium is built up and burnt effectively. In performing the flow rate spectral shift operation, as the power peaking in axial direction becomes large, the linear power density of the fuel rod becomes larger and the fuel rod is exposed to a more severe condition.
Accordingly, in order to decrease the linear power density of the fuel rod and to be sure to maintain thermal margin, it is necessary to reduce the power load per fuel rod by increasing number of the fuel rods in the fuel assembly by such method as changing the configuration of a fuel rods lattice from 8 lines by 8 rows to 9 lines by 9 rows etc.
In view of the two aspects described above, increasing of the number of fuel rods in the fuel assembly by changing the configuration of the fuel rods lattice and increasing of the H/U ratio by increasing of horizontal cross sectional area of the water rod or number of the water rods are a current trend in the fuel assembly for boiling water reactor.
For instance, in U.S. Pat. No. 4,781,885, a fuel assembly having a fuel rods lattice of 9 lines by 9 rows is disclosed, in which a large square water rod is installed at the central region which is equivalent to the 9 fuel rods arranged in a square lattice of 3 lines by 3 rows.
Further, in JP-A-1-196593 (1989), a fuel assembly having fuel rods in diamond lattice of which bearings to the internal wall of the channel box is 45.degree. is disclosed, in which a cruciform large water rod is installed at the central region which is equivalent to a region for 12 fuel rods.
More plutonium is built up generally at the upper region of the fuel assembly as described above, especially in case of the flow rate spectral shift operation, much plutonium are built up. When the quantity of plutonium built up at the upper region of the fuel assembly is increased so much, it becomes difficult to maintain surely the margin of the reactor shut down at cold shut down. The difficulty is caused by increasing of the infinite multiplication factor with disappearance of voids at the upper portion of the reactor core at the cold shut down.
In order to solve the problem, in JP-A-64-88292 (1989), a plurality of water rods are installed at least in symmetrical positions to the diagonal line of the fuel assembly and fuel rods having shorter length in axial direction than the other fuel rods (partial fuel rod) are installed at least at the position between the water rods. In the fuel assembly, the void fraction of coolant at the space above the partial fuel rods where the fuel is not located, namely vanishing rods, becomes zero at the cold shut down of the reactor. The portion of the vanishing rod acts as a large water rod with the other water rods at the cold shut down. Therefore, the portion of the vanishing rod has an excessive neutron moderating effect and reversely a large neutron absorbing effect at the cold shut down. As the result, the difference between the infinite multiplication factors during the operation of the reactor and during the cold shut down becomes small, and shut down margin of the reactor is increased. Further, in the case of installing of the partial fuel rods, an additional effect such as reducing of pressure loss at two phase flow portion in the fuel assembly under the reactor operation is brought.
As described above, increasing of the H/U ratio by increasing of the number of fuel rods, and further, increasing of horizontal cross sectional area of the water rods or the number of the water rods are the current tendency.
Under such trend of the current technical development, a trial is performed which is aimed at high burnup by increasing the enrichment further. Such increment of the enrichment aiming at the increasing of the discharge burnup necessitates further enlarging of the horizontal cross sectional area of the water rods in order to make the H/U ratio the most proper. Nevertheless, as the horizontal cross sectional area of the large water rod is enlarged according to U.S. Pat. No. 4,781,885 and JP-A-1-196593 (1989), the pressure loss of the reactor core is increased by narrowing of the area of the coolant flow path which is formed between the fuel rods.
The increasing of the pressure loss of the reactor core is a problem mainly in following points.
(1) When the pressure loss of the reactor core is increased, the capacity of the pump has to be increased in order to compensate the increment. If the maximum flow in the reactor core is achieved by the maximum rotation of the pump under the condition without the increment of the pressure loss, the pump is not able to achieve the maximum flow in the reactor core when the pressure loss is increased.
(2) Stability is lowered by increasing of the pressure loss. That is, as the pressure loss of the two phase flow in the upper portion of the fuel assembly is larger than the pressure loss of the single phase flow portion, when entering flow to the fuel assembly is increased, the resistance at the two phase flow portion is increased in order to reduce the entering flow. When the entering flow is reduced, the resistance at the two phase flow portion is reduced and the entering flow is increased again. By repeating of the phenomena, vibration of the flow is caused in the fuel assembly, and the stability is lowered. The larger the pressure loss at the two phase flow portion is, the easier the vibration of the flow is caused.
On the other hand, the fuel assembly which is described in JP-A-64-88292 (1989) has a problem in aspect of fuel economy because optimization of the H/U ratio is not considered on the fuel assembly.
Further, in case of installing partial fuel rods, which has two advantages such as the reduction of pressure loss at two phase flow portion and the secureness of the reactor shut down margin into the fuel assembly for high burnup as described in JP-A-64-88292, fuel inventory is decreased. The reduction of the fuel inventory increases the number of reload fuel assemblies and causes problems such as increment of the number of generated spent fuel assemblies. One of the methods for solving the problem is enlarging the diameter of the fuel rod for keeping the same fuel inventory as before installing of the partial fuel rod. But the method causes another problem of increasing pressure loss which is accompanied with the reduction of flow path area for the coolant.